This invention relates to an automatic sewing machine in which a workpiece holder having a workpiece held thereon is moved horizontally relative to the sewing needle to form a seam on the workpiece and automatically ceases its operation at the completion of formation of the seam and which comprises a thread straining control device adapted to strain the upper and lower threads under control by moving the workpiece holder relative to the needle, an operation device adapted to position the needle drop point in a position not to interfere with the insertion and removal of the workpiece into and from the sewing zone on the sewing machine bed and a display device adapted to notify the completion of computation of holder movement amount data to the operator.
In the conventional automatic sewing machines of the type referred to above, as shown in FIG. 6 of the accompanying drawings, during the time period from the timing t.sub.0 when the needle rises from a position below the throat plate towards a position above the throat plate to the timing t.sub.1, when the needle descends from the rised position towards the position below the throat plate, the workpiece holder has moved by a predetermined amount. However, in order to strain the threads to a proper degree, it has been designed that the termination timing of the workpiece feed (movement of the workpiece holder) is brought close to the above-mentioned timing t.sub.1. The reason is that when the needle 4 is high above the workpiece W at the completion of feed of the workpiece W as shown in FIG. 2, the length of the threads extending between the needle drop point for one stitch and the next needle drop point for the stitch immediately after the first-mentioned stitch is excessively increased. Thus, when the needle descends down to interconnect the threads and then the thread take-up lever rises, the rising movement of the lever does not sufficiently absorb slack in the upper thread and thus, the upper thread is not properly strained. Therefore, it has been proposed to coincide the timing of the workpiece feed with the timing of the termination of full descending movement of the needle 4 (feeding and straining).
In the above-mentioned prior art arrangement, assuming that the timing of workpiece feed completion is constant, when the workpiece has a relatively great thickness, the needle tends to contact the workpiece before the workpiece feed terminates resulting in the workpiece getting damaged. On the other hand, when the workpiece is relatively thin, the tension in the thread or threads is insufficient. Thus, it is necessary to adjust the timing of workpiece feed completion depending upon the thickness of a workpiece to be sewn. However, in the prior art arrangement, it requires a highly skilled hand for manipulating the internal mechanism of the sewing machine such as loosening the screws to allow the adjusting member to move by a predetermined amount for attaining the adjustment, the adjusting procedure becomes complicate and requires a prolonged time period.
As the automatic sewing machine referred to herein, various cycle sewing machines are known such as bar-tack, button hole darning, button fixing and embroidary sewing machines. Particularly, of late, electronic control sewing machines have been developed in which a pair of stepping motors are operatively conncted to a workpiece holder holding a workpiece thereon and the stepping motors are controlled based on informations stored in an information carrier such as a floppy disk whereby a seam of predetermined shape can be formed. In such a sewing machine, in the co-ordinates X, Y showing the position of a workpiece holder having the origin O.sub.1 as shown in FIG. 9, when a pattern A - B - C - D is sewn from the needle drop point A for the first stitch, the needle moves from an optional point K through the origin O.sub.1 to the needle drop point A for the first stitch and thus, the needle interferes with the insertion and removal of the workpiece into and from the sewing zone on the sewing machine.
In another prior art cycle sewing machine, a pair of stepping motors are operatively connected to a workpiece holder holding a workpiece thereon to drive the holder in X, Y directions, respectively, and movement data relating to movement of the holder are given by a floppy disk or the like to a memory. In such a sewing machine, a switch adapted to set numeral values is provided on the framework of the sewing machine so that the operator can optionally set data relating to selection of sewing patterns and magnification and reduction rates of the selected sewing patterns and movement amount of the workpiece holder for one stitch are computed based on data from the floppy disk.
In the prior art sewing machine described just above, the computation is performed during the time period from the timing at which the needle comes up out of the workpeice to the timing at which the needle pierces through the workpiece again for each stitch. However, such computation procedure requires a rather long time for each stitch and decreases the sewing operation speed.
In order to eliminate the drawbacks, it has been proposed that data relating to movement amount of seams of one cycle are previously computed by data from the floppy disk and values set by the switch prior to commencement of sewing, the computed result is stored in a memory means such as RAM or the like and after data for one cycle have been stored, sewing operation on the sewing machine is then commenced based on the data whereby sewing speed is increased because the time for computing movement amount data is not increased in the sewing time.
However, in the arrangement described just above, when sewing data to be computed are complication such as arcuates requiring a rather long time for computation, since the operator can not positively confirm when the computation terminated, the operator is at a loss as to when the operator actuates the sewing machine.